Reduction system and method for high-melting point metal oxides, using liquid metal crucible

ABSTRACT

The present disclosure provides a system and a method for reducing metal oxide to metal M1.

BACKGROUND Technical Field

The present disclosure relates to a system and a method for reducing a high-melting-point metal oxide using a liquid metal crucible.

Description of the Related Art

When a metal typically known in the art is referred to as any metal “M”, the metal M may be obtained by reducing a raw material such as an oxide or halide. Among the methods for producing the desired metal M, a method, which is relatively well-known and most widely commonly used in the art, is the so-called Kroll process.

Typically, the Kroll process may be summarized as a process in which molten magnesium is used as a reducing agent and a chloride of the desired metal M, such as titanium chloride or zirconium chloride, is added thereto and reduced to titanium or zirconium. In this regard, more details of the Kroll process may be found in U.S. Pat. No. 5,035,404.

Since this Kroll process is a process that uses chloride as a raw material, chlorine gas and magnesium chloride are generated as by-products during the process. Among these by-products, chlorine gas is regarded as a representative problem of the Kroll process, which is an environmental problem that causes fatal problems to the human body, and magnesium chloride causes process problems such as rapid corrosion of reaction vessels called cells, melting furnaces or crucibles or the like.

As such, the Kroll process requires an additional device to overcome environmental regulations, and involves frequent replacement of reaction vessels, resulting in high costs for operating the process.

In another aspect, the metal obtained through the Kroll is in the form of a sponge including a large number of pores, and thus it is very difficult to control oxygen that may exist in the metal. In other words, the Kroll process has limitations in obtaining high-purity metals.

Meanwhile, a method of preparing a desired metal alloy using CuCa or NiCa as a reducing agent and then electrorefining the same is being considered to overcome the disadvantages of the Kroll process.

However, the above method has a problem in that a reducing agent having strong reducing power directly contacts a portion of the reactor due to the characteristics of the process system, causing corrosion of the reaction vessel, similar to the Kroll process. There is also a method of minimizing the influence of the reducing agent by making a reaction vessel composed of W or Mo metal, but the unit cost of the reaction vessel itself is so high that it causes an increase in the overall production cost. In addition, in order to continuously perform the process, it is necessary to remove CaO, which is a by-product generated in the reduction reaction, and a large amount of a flux is required to remove the by-product. This also has a problem of causing an increase in the overall production cost.

Therefore, at present, there is a demand for a completely new technology capable of not only overcoming the problems of the Kroll process at once but also easily operating the process for obtaining the desired metal M while making it possible to obtain a large amount of the metal with high purity.

BRIEF SUMMARY

An object of the present disclosure is to provide a technology capable of overcoming the above-described problems.

The present disclosure provides a system optimized for obtaining a desired metal from a metal oxide without using a metal chloride or using chloride as a flux, and a method capable of producing this metal. Accordingly, the present disclosure is capable of resolving the environmental problems of the above-described Kroll process and the cost problems caused by cell corrosion.

Furthermore, the present disclosure provides a technology capable of obtaining a large amount of high-purity metal while facilitating the operation of the process according to the following aspects.

In one aspect of the present disclosure, a system and method provided in the present disclosure are characterized by using a liquid metal crucible that includes a liquid metal alloy of metal M¹ and metal M² forming a eutectic phase with each other.

The use of such a liquid metal crucible may significantly reduce energy consumption, leading to cost reduction. This because when a metal oxide included in a raw material module is reduced to metal M¹, the melting point of metal M¹ is lowered by a eutectic reaction so that electrolytic reduction may be effectively performed at a relatively low temperature.

In addition, in the above system and method, a liquid alloy (M¹ and M² form a liquid metal alloy) is obtained by a eutectic reaction, and thus the metal alloy itself may be used as a final product. Since the final product derived from such a liquid alloy has a significantly smaller specific surface area that may be in contact with oxygen than the sponge-type product of the Kroll process, the system and method of the present disclosure may minimize the problem of oxygen contamination of the product.

Alternatively, metal M¹ may be obtained by electrorefining the obtained metal alloy. The liquid alloy thus obtained may be completely isolated from an environment in which oxygen may exist, and thus contamination thereof by oxygen may be significantly prevented. That is, according to the above aspect, it is possible to obtain a high-purity metal alloy and metal M¹.

In another aspect of the present disclosure, in the system and method provided in the present disclosure, raw materials, for example, an oxide containing a desired metal, a reducing agent, and an alloying metal, form a module like a single part, and the system and method are characterized by using such a raw material module. In a process of continuously introducing a plurality of raw materials into a cell, like the Kroll process, the raw materials are likely to be oxidized or contaminated with oxygen before being introduced. However, the raw material module according to the present disclosure includes a structure treated to be prevented from oxidation, and thus has a more enhanced oxygen barrier effect compared to the Kroll process. Accordingly, the metal alloy and metal obtained according to the present disclosure may have a remarkably low oxygen content. In other words, according to the present disclosure, it is possible to obtain a high-purity metal alloy and metal having little oxygen.

According to these aspects, it is possible to easily producer a metal having excellent quality while overcoming the conventional problems. Thus, the technical basis for the implementation of the present disclosure will be described in detail below.

In one example embodiment of the present disclosure, a system for reducing a metal oxide to metal M¹ is provided.

A system according to one example embodiment of the present disclosure may include: a cell; a liquid metal crucible accommodated at the bottom of the cell and including a liquid metal alloy of metal M¹ and metal M² forming a eutectic phase with each other; a liquid flux accommodated in the cell while forming a layer on the liquid metal crucible without being mixed with the liquid metal crucible; and a solid raw material module including a metal oxide, metal M², and reducing metal M³, wherein the metal oxide is reduced to metal M¹ by reaction with reducing metal M³ while the solid raw material module reaches the liquid metal crucible and is melted, and the reduced metal M¹ and metal M² are continuously incorporated into the liquid metal crucible while forming a liquid metal alloy.

In one specific example embodiment, the system may further include an electrorefining part configured to collect and electrorefine the liquid metal alloy formed by the reduced metal M¹ and metal M² to obtain metal M₁.

In one specific example embodiment, the metal oxide may include at least one selected from the group consisting of M¹ _(x)O_(z) and M¹ _(x)M³ _(y)O_(z), wherein x and y are each a real number ranging from 1 to 3, and z is a real number ranging from 1 to 4.

In one specific example embodiment, the solid raw material module may include: a core layer including the metal oxide and the reducing metal M³; and a shell layer composed of metal M² surrounding the core layer.

In one specific example embodiment, the solid raw material module may be a multilayer structure including: a core layer including the metal oxide; and a shell layer coated to surround the outer surface of the core layer, wherein the shell layer may include an alloy phase composed of metal M² and metal M³.

In one specific example embodiment, the solid raw material module may be configured to descend vertically within the cell until it reaches the liquid metal crucible through the flux, and the solid raw material module may descend at a rate of a distance corresponding to 0.1% to 10% of the depth of the cell per min.

In one specific example embodiment, when the metal oxide is reduced to metal M¹ by reaction with reducing metal M³ while the solid raw material module is melted, oxide M³ _(a)O_(b) may be produced, and the oxide M³ _(a)O_(b) may have a lower specific gravity than that of the flux. Here, a and b are each a real number ranging from 1 to 3.

In one specific example embodiment, the oxide M³ _(a)O_(b) may float on the flux due to a density difference to form a by-product layer.

In one specific example embodiment, as the process progresses, the liquid metal alloy may be continuously collected through the bottom of the cell, and the by-product layer may be continuously collected through the top of the cell, thereby enabling a continuous process.

In one specific example embodiment, the system may further include a recycling part configured to collect the byproduct layer and mix the same with M¹ _(x)O_(z) to produce M¹ _(x)M³ _(y)O_(z).

In one specific example embodiment, the reaction between the metal oxide and the reducing metal may be performed in an inert gas atmosphere and/or air.

In one specific example embodiment, the core layer may be composed of a powder mixture including the metal oxide powder and the reducing metal M³ powder.

In one specific example embodiment, the core layer may have a multilayer structure including: a first core composed of the metal oxide; and a second core coated to surround the outer surface of the first core and composed of metal M³.

In one specific example embodiment, the solid raw material module may further include an oxidation-preventing layer surrounding the shell layer and serving to prevent oxidation of metal included in the core layer and/or the shell layer.

In one specific example embodiment, the oxidation-preventing layer may include at least one selected from the group consisting of LiF, MgF₂, CaF₂, BaF₂, CaCl₂, MgCl₂, MgO, CaO, BaO, Al₂O₃ and SiO₂.

In one specific example embodiment, metal M¹ may be one selected from the group consisting of Ti, Zr, Hf, W, Fe, Ni, Zn, Co, Mn, Cr, Ta, Ga, Nb, Sn, Ag, La, Ce, Pr, Nd, Nb, Pm, Sm, Eu, Al, V, Mo, Gd, Tb, Dy, Ho, Er, Tm, Yb, Ac, Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md and No, metal M² may be at least one selected from the group consisting of Cu, Ni, Fe, Sn, Zn, Pb, Bi, Cd, and alloys thereof, and metal M³ may be at least one selected from the group consisting of Ca, Mg, Al, and alloys thereof.

In one example embodiment of the present disclosure, a method of reducing and refining a metal oxide to metal M¹ is provided.

A method according to one example embodiment of the present disclosure may include: providing a cell; introducing a liquid flux into the cell; introducing metal M¹ and metal M² forming a eutectic phase with each other, thereby producing a liquid metal crucible having a specific gravity higher than that of the flux and accommodated in the cell while forming a layer under the flux without being mixed with the flux; moving a solid raw material module including a metal oxide, metal M² and reducing metal M³ to the cell until it reaches the liquid metal crucible through the flux; and obtaining a liquid metal alloy including metal M¹ derived from the metal oxide of the solid raw material module and metal M².

In one specific example embodiment, in the method, oxide M³ _(a)O_(b) may be produced as a by-product in moving the solid raw material module and/or obtaining the liquid metal alloy, and the oxide M³ _(a)O_(b) may have a lower specific gravity than that of the flux, and the method may further include continuously collecting the by-product M³ _(a)O_(b) forming a layer on the flux, and adding and mixing M¹ _(x)O_(z) with the collected M³ _(a)O_(b), thereby producing a metal oxide expressed as M¹ _(x)M³ _(y)O_(z) derived from the by-product M³ _(a)O_(b) and the added M¹ _(x)O_(z).

In one example embodiment of the present disclosure, three is provided a metal alloy or metal produced by the method according to the above-described example embodiment, wherein the metal alloy may have a residual reducing metal M³ content of 0.1 wt % or less, specifically 0.01 wt % or less, more specifically 0.001 wt % or less, based on the total weight of the metal alloy, and an oxygen content of 1,200 ppm or less, specifically 1,000 ppm or less, more specifically 990 ppm or less.

Technical Benefits

The present disclosure provides a system optimized for obtaining a desired metal from a metal oxide without using a metal chloride or using chloride as a flux, and a method for producing this metal. Accordingly, the present disclosure is capable of resolving the environmental problems of the above-described Kroll process and the cost problems caused by cell corrosion.

The system and method provided in the present disclosure are characterized by using a liquid metal crucible that includes a liquid metal alloy of metal M¹ and metal M² forming a eutectic phase with each other. The use of such a liquid metal crucible may significantly reduce energy consumption, leading to cost reduction. This because when a metal oxide included in a module, which is a raw material, is reduced to metal M¹, the melting point of metal M¹ is lowered by a eutectic reaction so that electrolytic reduction may be effectively performed at a relatively low temperature.

In addition, in the above system and method, a liquid alloy (M¹ and M² form a liquid metal alloy) is obtained by a eutectic reaction, and thus the metal alloy itself may be used as a final product. Since the final product derived from such a liquid alloy has a significantly smaller specific surface area that may be in contact with oxygen than the sponge-type product of the Kroll process, the system and method of the present disclosure may minimize the problem of oxygen contamination of the product.

The liquid alloy thus obtained may be completely isolated from an environment in which oxygen may exist, and thus contamination thereof by oxygen may be significantly prevented. That is, according to the foregoing, it is possible to obtain a high-purity metal alloy and metal M¹.

In the system and method provided in the present disclosure, raw materials, for example, an oxide containing a desired metal, a reducing agent, and an alloying metal, form a module like a single part, and the system and method are characterized by using such a raw material module. In a process of continuously introducing a plurality of raw materials into a cell, like the Kroll process, the raw materials are likely to be oxidized or contaminated with oxygen before being introduced. However, the raw material module according to the present disclosure includes a structure treated to be prevented from oxidation, and thus has a more enhanced oxygen barrier effect compared to the Kroll process. Accordingly, the metal alloy and metal obtained according to the present disclosure may have a remarkably low oxygen content. In other words, according to the present disclosure, it is possible to obtain a high-purity metal alloy and metal having little oxygen.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic view of a system according to one example embodiment of the present disclosure;

FIG. 2 depicts photographs showing a process of preparing a raw material module including MgTiO₃ as a metal oxide according to one example embodiment of the present disclosure;

FIG. 3 is a graph showing the results of XRD analysis of the raw material module prepared as shown in FIG. 2 ;

FIG. 4 is a schematic vertical sectional view of a raw material module according to one example embodiment of the present disclosure;

FIG. 5 is a schematic vertical sectional view of a raw material module according to another example embodiment of the present disclosure;

FIG. 6 is a schematic vertical sectional view of a raw material module according to still another example embodiment of the present disclosure;

FIG. 7 is a photograph of a raw material module;

FIG. 8 is another photograph of a raw material module;

FIG. 9 depicts photographs showing the results of comparing weight before and after removing the flux from the alloy ingot produced in an Example;

FIG. 10 is a table showing the results of performing elemental analysis of the inside of an alloy, produced in an Example, by energy dispersive spectrometry (EDS) after cutting the alloy; and

FIG. 11 is a table showing the results of measuring the content of oxygen present in the alloy using ELTRA ONH2000.

DETAILED DESCRIPTION

Hereinafter, the intentions, operations and effects of the present disclosure will be described in detail through specific descriptions and examples to assist in understanding the example embodiments of the present disclosure and to implement these example embodiments. However, the following descriptions and example embodiments are presented as examples to assist in understanding the present disclosure as described above, and the scope of the invention is neither defined thereby nor limited thereto.

Prior to the detailed description of the present disclosure, it should be noted that the terms or words used in the present specification and claims should not be construed as being limited to typical meanings or dictionary definitions, but should be interpreted as having meanings and concepts relevant to the technical scope of the present disclosure, based on the principle according to which the inventors can appropriately define the meaning of the terms to describe their invention in the best manner.

Accordingly, it should be understood that the example embodiments described in the present specification and the configurations shown in the drawings are merely the most preferred example embodiments, but not cover all the technical spirits of the present disclosure, and thus there may be various equivalents and modifications capable of replacing them at the time of filing the present disclosure.

In the present specification, singular expressions include plural expressions unless the context clearly indicates otherwise. In the present specification, it should be understood that terms such as “include” and “have” are intended to denote the existence of mentioned characteristics, numbers, operations, components, or combinations thereof, but do not exclude the possibility of existence or addition of one or more other characteristics, numbers, operations, components, or combinations thereof.

The term “charging” as used in the present specification may be used interchangeably with the term “feeding”, “introducing”, “flowing in”, or “injection”, and may be understood to mean sending or putting any material, such as a raw material, into a place where it is needed.

Hereinafter, the present disclosure will be described in detail in the order of a system for reduction to metal M¹, a method for reduction to metal M¹, and examples.

1. System for Reduction to Metal M¹

In one specific example embodiment, a system for a metal oxide to metal M¹ according to the present disclosure is schematically shown in FIG. 1 . Referring to FIG. 1 , the system according to the present disclosure may include: a cell 400; a liquid metal crucible 100 accommodated at the bottom of the cell 400 and including a liquid metal alloy of metal M¹ and metal M² forming a eutectic phase with each other; a liquid flux 200 accommodated in the cell while forming a layer on the liquid metal crucible 100 without being mixed with the liquid metal crucible, so as to block oxygen and reaction by-products from flowing into the liquid metal crucible 100; and a solid raw material module 300 including a metal oxide, metal M², and reducing metal M³, wherein the metal oxide may be reduced to metal M¹ by reaction with the reducing metal M³ while the solid raw material module reaches the liquid metal crucible and is melted, and the reduced metal M¹ and metal M² may be continuously incorporated into the liquid metal crucible while forming a liquid metal alloy.

In the system of the present disclosure, the desired metal M¹ is not particularly limited, but may be specifically one selected from the group consisting of Ti, Zr, Hf, W, Fe, Ni, Zn, Co, Mn, Cr, Ta, Ga, Nb, Sn, Ag, La, Ce, Pr, Nd, Nb, Pm, Sm, Eu, Al, V, Mo, Gd, Tb, Dy, Ho, Er, Tm, Yb, Ac, Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md and No. More specifically, the desired metal M¹ may be one selected from the group consisting of Ti, Zr, W, Fe, Ni, Zn, Co, Mn, Cr, Ta, Er and No, and more specifically, may be one selected from the group consisting of Ti, Zr, W, Fe, Ni, Zn, Co, Mn, and Cr. Even more specifically, it may be Ti, Zr or W.

In the system of the present disclosure, metal M² is not particularly limited as long as it can form a liquid metal alloy by a eutectic reaction with metal M¹. Specifically, metal M² may be at least one selected from the group consisting of Cu, Ni, Fe, Sn, Zn, Pb, Bi, Cd, and alloys thereof, and more specifically, may be Cu, Ni, or an alloy thereof.

In the system of the present disclosure, metal M³ is not particularly limited as long as it can reduce the metal oxide to M¹. Specifically, metal M³ may be at least one selected from among Ca, Mg, Al, and alloys thereof, and more specifically, may be Ca or Mg, in particular, Mg.

In the system of the present disclosure, the metal oxide may include at least one selected from the group consisting of M¹ _(x)O_(z) and M¹ _(x)M³ _(y)O_(z), wherein x and y are each a real number ranging from 1 to 3, and z is a real number ranging from 1 to 4.

In one specific example embodiment, the metal oxide may include one or a combination of two or more selected from the group consisting of ZrO₂, TiO₂, MgTiO₃, HfO₂, Nb₂O₅, Dy₂O₃, Tb₄O₇, WO₃, Co₃O₄, MnO, Cr₂O₃, MgO, CaO, Al₂O₃, Ta₂O₅, Ga₂O₃, Pb₃O₄, SnO, NbO and Ag₂O, without being limited thereto.

The system according to the present disclosure is different from the conventional Kroll process in that it uses a metal oxide instead of a metal chloride as a raw material. Raw materials usually found in nature include an oxide of metal M¹, and in order to use this metal oxide in the Kroll process, a pretreatment process of substituting the metal oxide with a chloride needs to be performed. If this pretreatment process is performed, it itself will cause an increase in process cost. Moreover, hydrochloric acid is used in the pretreatment process of substituting the metal oxide with a chloride, and in this case, corrosion of production equipment may be promoted due to the strong acidity of the hydrochloric acid, and toxic chlorine gas may be generated during the process, which may cause environmental problems. The system according to the present disclosure has advantages over the Kroll process in that it does not require the pretreatment process of substituting the metal oxide with a chloride, and thus it incurs a lower cost than the Kroll process cost and does not cause environmental problems.

The cell 400 is preferably made of a material that has a high melting point in terms of durability and does not cause side reactions with the flux and the liquid metal crucible. The material of the cell may include at least one selected from the group consisting of MgO, Cr₂O₃, Al₂O₃, SiO₂, CaO, SiC, WO₃, W, C, and Mo, without being limited thereto.

The cell 400 may include a tapping hole 410 for continuously tapping the liquid metal alloy produced at the bottom thereof.

As used herein, the term “liquid metal crucible” may refer to a reaction region capable of providing an environment in which at least one metal may be accommodated in, for example, the cell in a molten state while forming one layer, and the raw material module of the present disclosure may be melted within and the layer surface of the melted metal so that the metal oxide may be reduced.

The system according to the present disclosure has the advantage of using this liquid metal crucible.

Specifically, in the system of the present disclosure, as the liquid metal crucible including the liquid metal alloy of metal M¹ and metal M² forming a eutectic phase with each other is used, melting point of metal M¹ may be lowered by the eutectic reaction while the metal oxide of the raw material module is reduced to metal M¹, and thus electrolytic reduction may be effectively performed at a relatively low temperature. That is, in the system of the present disclosure, it is possible to obtain a liquid metal alloy while operating a liquid metal crucible in which metals are melted, and it is possible to operate the process at a lower temperature than the melting point of metal M¹, thereby significantly reducing energy consumption. This temperature may vary depending on the types of M¹ and M², but may preferably be 900° C. to 1,600° C.

What is also noteworthy about the liquid metal crucible is that a liquid alloy (M¹ and M² form a liquid metal alloy) may be obtained based on the eutectic reaction induced in the system of the present disclosure, and the metal alloy itself may be used as a final product. M¹ is often used industrially in the form of an alloy. When M¹ can be produced as only a single metal as in the conventional Kroll process, a post-processing process for forming an alloy with another metal may be required. However, the present disclosure has high process efficiency in that it is possible to obtain a final product in the form of a metal alloy of M¹ and M² through reduction reaction without the above-described post-treatment process. In addition, when the liquid metal crucible is formed, there are advantages in that it is possible to adjust the ratio between M¹ and M², it is also possible to adjust the ratio between the metal oxide and M² in the module, and thus it is possible to the ratio between M¹ and M² in the metal alloy as a final product. It is to be understood that, if necessary, metal M¹ may be obtained by electrorefining the obtained metal alloy, and a method known in the art may be used for such electrolytic refining.

The liquid alloy obtained according to the present disclosure may be completely isolated from an environment in which oxygen may exist, and thus contamination thereof by oxygen may be significantly prevented. That is, according to the above aspect, it is possible to obtain a high-purity metal alloy and metal M¹.

Furthermore, the use of the liquid metal crucible including the liquid metal alloy of metal M¹ and metal M² forming a eutectic phase with each other has an advantage in that, even if the metal oxide contained in the raw material module is a material that is difficult to electrolytically reduce to metal, it is possible to more easily reduce the metal oxide in the raw material module using the standard oxidation-reduction potential difference between the liquid metal alloy and the desired metal M¹. That is, when metal M² having a more positive standard reduction potential that of metal M¹ is used, the standard reduction potential value of M¹ may move in a positive direction due to the liquid metal crucible, so that electrolytic reduction of the metal may be more easily achieved.

In one major aspect of the present disclosure, in the system of the present disclosure, a metal oxide as a raw material, another metal as a reducing agent, and other additive metals are not introduced, but they constitute a raw material module like a single part, and based on this raw material module, it is possible to obtain a higher-quality metal alloy and metal.

For example, in a process of continuously introducing a plurality of raw materials into a cell, like the Kroll process, the raw materials are likely to be oxidized before being introduced, but the raw material module according to the present disclosure includes a structure treated to be prevented from oxidation, and thus has a more enhanced oxygen barrier effect compared to the Kroll process. Accordingly, the metal alloy and metal obtained according to the present disclosure may have a significantly low oxygen content.

Additionally, as described above, the system of the present disclosure includes a configuration for doubly blocking oxygen, because the flux serves as a barrier layer for blocking oxygen, and the raw material module itself can also block oxygen. Accordingly, the system of the present disclosure has significant advantages in that it is possible to perform the reaction between the metal oxide and the reducing metal under an inert gas atmosphere, which has been commonly recognized in the art, and it is also possible to perform the reaction between the metal oxide and the reducing metal in air. Even if the system of the present disclosure is operated in air, the metal alloy and metal produced thereby are of high purity with little oxygen. This will be clearly demonstrated through examples to be described later. In some cases, in the system of the present disclosure, the reaction between the metal oxide and the reducing metal may be performed in a combination of an inert gas atmosphere and a normal air atmosphere.

In addition, the system of the present disclosure uses a raw material module in which raw materials necessary for the reaction are gathered into one body, and thus has an advantage in that it is easy to induce the reaction at the most optimal location in the cell.

Photographs showing a process of preparing a raw material module according to the present disclosure are depicted in FIG. 2 , and FIG. 4 shows a schematic view of a raw material module according to one example embodiment of the present disclosure. Incidentally, the photographs shown in FIG. 2 are only for helping understanding of the preparation of the raw material module as a non-limiting example, and the scope of the present disclosure is not limited thereto.

Referring to FIG. 4 , the solid raw material module 300 may include: a core layer 30 including the metal oxide 10 and the reducing metal M³ 30; and a shell layer 320 composed of metal M² surrounding the core layer 310. As shown in FIG. 4 , the core layer 310 may have a multilayer structure including: a first core 311 composed of the metal oxide 10; and a second core 312 coated to surround the outer surface of the first core 311 and composed of the reducing metal M³ 30.

In some cases, a raw material module 300 a as shown in FIG. 5 in which a core layer is shown as another example of the present disclosure may be used. The core layer 310 a of the raw material module 300 a may be composed of a powdery mixture including metal oxide powder 10 a and reducing metal M³ powder 30 a.

Alternatively, the structure shown in FIG. 6 may also be preferable as the raw material module. Referring to FIG. 6 , the raw material module 300 b is similar to the above-described example in terms of a multilayer structure, but is different from the above-described example in that it includes the core layer 310 b including the metal oxide 10 b and the shell layer 320 b coated to surround the outer surface of the core layer 310 b, wherein the shell layer 320 b is a coating layer including an alloy phase 330 b composed of the metal M² 20 b and the metal M³ 30 b.

The above-described solid raw material module may further include an oxidation-preventing layer (330 in FIG. 4 ) surrounding the shell layer and serving to prevent oxidation of metal included in the core layer and/or the shell layer by blocking oxygen from contacting the metal. The oxidation-preventing layer 330 may include at least one selected from the group consisting of LiF, MgF₂, CaF₂, BaF₂, CaCl₂, MgCl₂, MgO, CaO, BaO, Al₂O₃ and SiO₂, but the scope of the present disclosure is not limited thereto. Although the oxidation-preventing layer is shown only in FIG. 4 , it is to be understood that the oxidation-preventing layer may also be applied to the other example embodiments shown in FIGS. 5 and 6 .

This solid raw material module may be configured to descend vertically within the cell until it reaches the liquid metal crucible through the flux. The solid raw material module may descend at a rate of a distance corresponding to 0.1% to 10% of the depth of the cell per min.

When the descending direction of the solid raw material module is set as an imaginary axis, rotating the solid raw material module about the axis is preferable in terms of stirring the liquid metal crucible and improving reactivity thereby. The rotation may be performed during the descent into the cell and/or until completion of the descent.

Accordingly, the system according to the present disclosure may further include a rotation unit to which the solid raw material module is mounted and which rotates the same.

Meanwhile, the solid raw material module descends to the liquid metal crucible and is melted, and at the same time or partially simultaneously, the metal oxide and the reducing metal M³ react to reduce the metal oxide to M¹, and the reduced M¹ forms a liquid metal alloy with M² contained in the solid raw material module.

As an example, when M¹ is Ti, the metal oxide (M¹ _(x)O_(z)) is TiO₂, M² is Ni, and M³ is Mg, according to Reaction Formulas 1-1 and 1-2 below, the metal oxide may be reduced to metal Ti, and then an M³ oxide (M³ _(a)O_(b)) may be separated while the liquid metal alloy TiNi is obtained.

2Mg+TiO₂->Ti+2MgO  [Reaction Formula 1-1]

Ti+Ni+2MgO→TiNi (alloy)+2MgO (separated)  [Reaction Formula 1-2]

As another example, when M¹ is Ti, the metal oxide (M¹ _(x)M³ _(y)O_(z)) is MgTiO₃, M² is Ni, and M³ is Mg, according to Reaction Formulas 2-1 and 2-2 below, the metal oxide may be reduced to metal Ti, and then an M³ oxide (M³ _(a)O_(b)) may be separated while the liquid metal alloy TiNi is obtained.

2Mg+MgTiO₃->Ti+3MgO  [Reaction Formula 2-1]

Ti+Ni+3MgO→TiNi (alloy)+3MgO (separated)  [Reaction Formula 2-2]

M₃aO_(b) produced according to the above-described reaction is a kind of by-product and may have a lower specific gravity that that of the flux of the present disclosure. The M³ _(a)O_(b) may float on the flux due to its density difference from the flux to form a by-product layer. Therefore, the by-product M³ _(a)O_(b) does not mix with the liquid metal crucible present as a layer under the flux and with the formed liquid metal alloy. In addition, the by-product layer may serve to prevent the flux from being lost by vaporization while being positioned on the flux, and to prevent oxygen in the air from penetrating into the reactor.

Meanwhile, since the by-product floats on the cell as the liquid metal alloy is produced, the by-product needs to be continuously removed from the cell in order to continuously perform the process in the cell having a limited volume. Thus, the system according to the present disclosure may be configured to enable a continuous process by using this by-product. Specifically, the system of the present disclosure may further include a recycling device that continuously collects the layered by-product floating on the flux through the top of the cell and mixes the collected by-product with, for example, M¹ _(x)O_(z) to produce metal oxide M¹ _(x)M³ _(y)O_(z). At this time, when the produced M¹ _(x)M³ _(y)O_(z) is used, the reduction reaction rate may be further increased compared to when M¹ _(x)O_(z) is used.

The flux preferably has a specific gravity which is intermediate between the specific gravity of the liquid metal crucible and the specific gravity of the by-product M³ _(a)O_(b) so as to prevent the liquid metal crucible and the by-product M³ _(a)O_(b) from mixing with each other, and at the same time, the flux is preferably insoluble in the by-product M³ _(a)O_(b). The flux is preferably a material such as a non-chlorine-based material, which does not cause environmental problems while being capable of preventing prevent oxygen from penetrating into the liquid metal crucible and the liquid metal alloy containing the desired metal M¹.

This flux may include a molten halide salt of at least one metal selected from the group consisting of alkali metals and alkaline earth metals, but does not contain chloride. More specifically, the flux in the system of the present disclosure may be a molten halide salt of at least one metal selected from the group consisting of alkali metals including Li, Na, K, Rb, and Cs, and alkaline earth metals including Mg, Ca, Sr, and Ba. In this case, the halide may include fluoride, bromide, iodide, or a mixture thereof.

In the system of the present disclosure, the flux may also be present in an amount of 10 wt % to 50 wt %, specifically 10 wt % to 20 wt %, more specifically 10 wt % to 15 wt %, even more specifically 12 wt % to 13 wt %, relative to the metal oxide involved in the overall reduction reaction, that is, the metal oxide contained in the raw material module and capable of being reduced to the desired metal M¹.

The flux may further contain, as an additive, at least one metal oxide selected from the group of alkali metals and alkaline earth metals. The content of the additive may be 0.1 to 25 wt % based on the total weight of the flux. The additive may include Li₂O, Na₂O, SrO, Cs₂O, K₂O, CaO, BaO, or a mixture thereof, without being limited thereto. The additive contained in the flux may enable easier reduction of the metal oxide contained in the raw material module.

The system according to the present disclosure may further include an electrorefining part configured to continuously collect the liquid metal alloy formed by M¹ and M² through the bottom of the cell and to electrorefine the collected liquid metal alloy to obtain metal M₁.

The electrorefining part may solidify the collected liquid metal alloy to obtain a solid metal alloy, and electrorefine the solid metal alloy, thereby recovering metal M¹ from the metal alloy.

In some cases, a flux that may remain in the liquid metal alloy may be removed before electrorefining of the solid metal alloy, and this removal may be achieved, for example, by heat-treating the liquid metal alloy in a vacuum or inert gas atmosphere, causing the flux to be removed by distillation. The distillation temperature (heat treatment temperature) is not particularly limited as long as it is a temperature equal to or higher than the melting point of the flux used in the system of the present disclosure, and it may be, for example, 780 to 1,000° C. In order to effectively prevent the liquid metal alloy from being oxidized again, it may be advantageous to carry out the distillation in a vacuum atmosphere and under an inert gas atmosphere.

The electrorefining part may include a flux including a molten halide salt of at least one metal selected from the group consisting of alkali metals and alkaline earth metals, independently of the flux used in the above-described reduction reaction.

2. Production Method

The method according to the present disclosure may include the operations of: providing a cell; introducing a liquid flux into the cell; introducing metal M¹ and metal M² forming a eutectic phase with each other, thereby producing a liquid metal crucible having a specific gravity higher than that of the flux and accommodated in the cell while forming a layer under the flux without being mixed with the flux; moving a solid raw material module including a metal oxide, metal M² and reducing metal M³ to the cell until it reaches the liquid metal crucible through the flux; and obtaining a liquid metal alloy including metal M¹ derived from the metal oxide of the solid raw material module and metal M².

In the operation of moving the solid raw material module, when the solid raw material module is melted when it reaches the liquid metal crucible. In this case, the metal oxide and the reducing metal M³ may react with each other to reduce the metal oxide to metal M¹, and the reduced metal M¹ and metal M² may be continuously incorporated into the liquid metal crucible while forming a liquid metal alloy.

In the method of the present disclosure, the desired metal M¹ is not particularly limited, but may be specifically one selected from the group consisting of Ti, Zr, Hf, W, Fe, Ni, Zn, Co, Mn, Cr, Ta, Ga, Nb, Sn, Ag, La, Ce, Pr, Nd, Nb, Pm, Sm, Eu, Al, V, Mo, Gd, Tb, Dy, Ho, Er, Tm, Yb, Ac, Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md and No. More specifically, the desired metal M¹ may be one selected from the group consisting of Ti, Zr, W, Fe, Ni, Zn, Co, Mn, Cr, Ta, Er and No, and more specifically, may be one selected from the group consisting of Ti, Zr, W, Fe, Ni, Zn, Co, Mn, and Cr. Even more specifically, it may be Ti, Zr or W.

In the method of the present disclosure, metal M² is not particularly limited as long as it can form a liquid metal alloy by a eutectic reaction with metal M¹. Specifically, metal M² may be at least one selected from the group consisting of Cu, Ni, Fe, Sn, Zn, Pb, Bi, Cd, and alloys thereof, and more specifically, may be Cu, Ni, or an alloy thereof.

In the method of the present disclosure, metal M³ is not particularly limited as long as it can reduce the metal oxide to M¹. Specifically, metal M³ may be at least one selected from among Ca, Mg, Al, and alloys thereof, and more specifically, may be Ca or Mg, in particular, Mg.

In the method of the present disclosure, the metal oxide may include at least one selected from the group consisting of M¹ _(x)O_(z) and M¹ _(x)M³ _(y)O_(z), wherein x and y are each a real number ranging from 1 to 3, and z is a real number ranging from 1 to 4.

In one specific example embodiment, the metal oxide may include one or a combination of two or more selected from the group consisting of ZrO₂, TiO₂, MgTiO₃, HfO₂, Nb₂O₅, Dy₂O₃, Tb₄O₇, WO₃, Co₃O₄, MnO, Cr₂O₃, MgO, CaO, Al₂O₃, Ta₂O₅, Ga₂O₃, Pb₃O₄, SnO, NbO and Ag₂O, without being limited thereto.

The method according to the present disclosure is different from the conventional Kroll process in that it uses a metal oxide instead of a metal chloride as a raw material. Raw materials usually found in nature include an oxide of metal M¹, and in order to use this metal oxide in the Kroll process, a pretreatment process of substituting the metal oxide with a chloride needs to be performed. If this pretreatment process is performed, it itself will cause an increase in process cost. Moreover, hydrochloric acid is used in the pretreatment process of substituting the metal oxide with a chloride, and in this case, corrosion of production equipment may be promoted due to the strong acidity of the hydrochloric acid, and toxic chlorine gas may be generated during the process, which may cause environmental problems. The method according to the present disclosure has advantages over the Kroll process in that it does not require the pretreatment process of substituting the metal oxide with a chloride, and thus it incurs a lower cost than the Kroll process cost and does not cause environmental problems.

The raw material module that is used in the method of the present disclosure may include: a core layer including a metal oxide and reducing metal M³; and a shell layer composed of metal M² surrounding the core layer. As shown in FIG. 4 , the core layer 310 may have a multilayer structure including: a first core composed of the metal oxide; and a second core coated to surround the outer surface of the first core and composed of the reducing metal M³. In some cases, as shown in FIG. 5 in which a core layer is shown as another example of the present disclosure. The core layer 310 a of the raw material module 300 a may be composed of a powdery mixture including metal oxide powder and reducing metal M³ powder.

Alternatively, the raw material module shown in FIG. 6 may also be used. This raw material module may include the core layer including the metal oxide and the shell layer coated to surround the outer surface of the core layer, wherein the shell layer may be a coating layer including an alloy phase composed of metal M² and metal M³.

The solid raw material module may further include an oxidation-preventing layer surrounding the shell layer and serving to prevent oxidation of metal included in the core layer and/or the shell layer by blocking oxygen from contacting the metal. The oxidation-preventing layer may include at least one selected from the group consisting of LiF, MgF₂, CaF₂, BaF₂, CaCl₂, MgCl₂, MgO, CaO, BaO, Al₂O₃ and SiO₂, but the scope of the present disclosure is not limited thereto.

In the operation of moving the solid raw material module, the solid raw material module may be descended at a rate of a distance corresponding to 0.1% to 10% of the depth of the cell per min, until it reaches the liquid metal crucible through the flux.

In the operation of obtaining a liquid metal alloy including metals M¹ and M², when the solid raw material module reaches the liquid metal crucible and is melted, the metal oxide may be reduced to metal M¹ by reaction with the reducing metal M³, and the reduced M¹ may form a liquid metal alloy with the metal M² contained in the solid raw material module.

As an example, when M¹ is Ti, the metal oxide (M¹ _(x)O_(z)) is TiO₂, M² is Ni, and M³ is Mg, according to Reaction Formulas 1-1 and 1-2 below, the metal oxide may be reduced to metal Ti, and then an M³ oxide (M³ _(a)O_(b)) may be separated while the liquid metal alloy TiNi is obtained.

2Mg+TiO₂->Ti+2MgO  [Reaction Formula 1-1]

Ti+Ni+2MgO→TiNi (alloy)+2MgO (separated)  [Reaction Formula 1-2]

As another example, when M¹ is Ti, the metal oxide (M¹ _(x)M³ _(y)O_(z)) is MgTiO₃, M² is Ni, and M³ is Mg, according to Reaction Formulas 2-1 and 2-2 below, the metal oxide may be reduced to metal Ti, and then an M³ oxide (M³ _(a)O_(b)) may be separated while the liquid metal alloy TiNi is obtained.

2Mg+MgTiO₃->Ti+3MgO  [Reaction Formula 2-1]

Ti+Ni+3MgO→TiNi (alloy)+3MgO (separated)  [Reaction Formula 2-2]

M₃aO_(b) produced according to the above-described reaction is a kind of by-product and may have a lower specific gravity that that of the flux of the present disclosure. The M³ _(a)O_(b) may float on the flux due to its density difference from the flux to form a by-product layer. Therefore, the by-product M³ _(a)O_(b) does not mix with the liquid metal crucible present as a layer under the flux and with the formed liquid metal alloy. In addition, the by-product layer may serve to prevent the flux from being lost by vaporization while being positioned on the flux, and to prevent oxygen in the air from penetrating into the reactor.

Meanwhile, since the by-product floats on the cell as the liquid metal alloy is produced, the by-product needs to be continuously removed from the cell in order to continuously perform the process in the cell having a limited volume. Thus, the method according to the present disclosure may be configured to use this by-product. Specifically, the method according to the present disclosure may further include the operation of continuously collecting the layered by-product (i.e., M³ _(a)O_(b)) floating on the flux through the top of the cell, and adding and mixing, for example, M¹ _(x)O_(z) with the collected M³ _(a)O_(b), thereby producing the metal oxide M¹ _(x)M³ _(y)O_(z) derived from the by-product M³ _(a)O_(b) and the added M¹ _(x)O_(z).

When M¹ _(x)M³ _(y)O_(z) obtained as described above is used, in some cases, the reduction reaction rate may be further increased compared to when M¹ _(x)O_(z) is used. The operation of producing M¹ _(x)M³ _(y)O_(z) may be performed at a temperature of 1,000° C. to 1,500° C., specifically 1,200° C. to 1,400° C., more specifically, 1,250° C. to 1,350° C.

The type of the flux is not particularly limited unless it is a chlorine-based material, and is preferably as defined in the previous example embodiment.

The method according to the present disclosure may further include, after the operation of obtaining the alloy including metals M¹ and M², the operation of obtaining metal M¹ by electrorefining the obtained alloy.

The operation of obtaining metal M¹ by electrorefining may be the operation of solidifying the obtained liquid metal alloy to obtain a solid alloy, and electrorefining the solid alloy, thereby recovering metal M¹ from the alloy.

3. Examples

Hereinafter, example will be described in detail with reference to FIGS. 3 to 5 and 6 a to 6 c, whereby the action and effect of the present disclosure will be demonstrated. However, the following examples are only presented as examples of the present disclosure, and the scope of the present disclosure is not defined thereby.

Example

The system shown in FIG. 1 was used. Flux MgF₂ (0.2 kg)-BaF₂ (1.5 kg) in a resistance heating furnace was weighed, introduced into a cell, and then heated to about 1,200° C. to form a flux layer.

Cu (20 g) and Ti (200 g) were weighed, introduced into the cell, and melted, thereby producing a liquid metal crucible positioned at the bottom of the cell and forming a layer under the flux.

As shown in FIG. 2 , 630 g of MgTiO₃ (average particle size of 300 μm) as a metal oxide and 250 g of Mg powder as metal M³ were mixed together, charged into a cylindrical copper container (250 g), and dried, thus preparing a raw material module. Incidentally, photographs of the actually prepared raw material module are shown in FIGS. 7 and 8 .

The prepared raw material module was charged into the cell and descended vertically at a rate of about 6 cm/min until it reached the layer of the liquid metal crucible. At this time, the module was rotated for 10 minutes to stir the flux and the liquid metal crucible. The melting and reduction reaction of the raw material module was performed for 2 hours, and a CuTi liquid metal alloy as a reaction product was collected through an outlet provided at the bottom of the cell and was solidified to finally obtain a CuTi alloy shown in FIG. 9 . In addition, after completion of the reaction, the crucible was cooled at a rate of −10° C./min to prevent damage to the crucible.

Experimental Example

The properties of the alloy obtained in Example 1 were evaluated using the following methods.

-   -   Recovery: 100−{(first weight−second weight)/second weight×100%}     -   Residual impurity content: the produced alloy was cut and the         inside of the alloy was analyzed by energy dispersive         spectrometry.     -   Oxygen content: the oxygen content in the alloy was measured         using an ELTRA ONH2000.

TABLE 1 Energy dispersive First Second Recov- spectrometry results Oxygen weight* weight** ery Ti Cu content (g) (g) (%) (wt %) (wt %) (ppm) Example 500 488.56 97.7% 40.29 59.71 980.35 *First weight: total weight of liquid metal crucible of CuTi initially charged into cell + stoichiometric reduction amount of Ti contained in metal oxide **Second weight: total weight of CuTi obtained

From the results in Table 1 above, it can be seen that the alloy of the Example, produced according to the present disclosure, exhibited a high recovery rate which is much higher than 90%, and contained oxygen as a contaminant at an extremely low level. The results for this low oxygen content are clearly demonstrated in FIG. 11 .

In addition, FIG. 10 shows the results obtained by cutting the alloy produced in the Example and analyzing the residual impurity content in the inside of the alloy by energy dispersion spectrometry. Referring to FIG. 10 , it can be seen that the alloy is composed only of the desired metal Ti and Cu, and Mg used as the reducing metal does not exist at all.

These experimental results suggest that, according to the present disclosure, it is possible to obtain a high-purity alloy which has a very low oxygen content and has no other impurities used in the process.

The various embodiments described above can be combined to provide further embodiments. All of the U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications, and publications to provide yet further embodiments.

These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure. 

1. A system for reducing a metal oxide to metal M¹, the system comprising: a cell; a liquid metal crucible accommodated at a bottom of the cell and comprising a liquid metal alloy of metal M¹ and metal M² forming a eutectic phase with each other; a liquid flux accommodated in the cell while forming a layer on the liquid metal crucible without being mixed with the liquid metal crucible; and a solid raw material module comprising a metal oxide, metal M², and reducing metal M³, wherein the metal oxide is reduced to metal M¹ by reaction with the reducing metal M³ while the solid raw material module reaches the liquid metal crucible and is melted, and the reduced metal M¹ and the metal M² are continuously incorporated into the liquid metal crucible while forming a liquid metal alloy.
 2. The system according to claim 1, further comprising an electrorefining part configured to collect and electrorefine the liquid metal alloy formed by the reduced metal M¹ and the metal M² to obtain metal M₁.
 3. The system according to claim 1, wherein the metal oxide comprises at least one selected from the group consisting of M¹ _(x)O_(z) and M¹ _(x)M³ _(y)O_(z), wherein x and y are each a real number ranging from 1 to 3, and z is a real number ranging from 1 to
 4. 4. The system according to claim 1, wherein the solid raw material module comprises: a core layer comprising the metal oxide and the reducing metal M³; and a shell layer composed of metal M² surrounding the core layer.
 5. The system according to claim 1, wherein: the solid raw material module is a multilayer structure comprising: a core layer comprising the metal oxide; and a shell layer coated to surround an outer surface of the core layer; and the shell layer comprises an alloy phase composed of the metal M² and the metal M³.
 6. The system according to claim 1, wherein: the solid raw material module is configured to descend vertically within the cell until it reaches the liquid metal crucible through the flux; and the solid raw material module descends at a rate of a distance corresponding to 0.1% to 10% of a depth of the cell per min.
 7. The system according to claim 1, wherein, when the metal oxide is reduced to the metal M¹ by reaction with the reducing metal M³ while the solid raw material module is melted, oxide M³ _(a)O_(b) is produced, and the oxide M³ _(a)O_(b) has a lower specific gravity than that of the flux, wherein a and b are each a real number ranging from 1 to
 3. 8.-10. (canceled)
 11. The system according to claim 1, wherein the reaction between the metal oxide and the reducing metal is performed in an inert gas atmosphere and/or air.
 12. The system according to claim 4, wherein the core layer is composed of a powder mixture comprising powder of the metal oxide powder and powder of the reducing metal M³.
 13. The system according to claim 4, wherein the core layer is a multilayer structure comprising: a first core composed of the metal oxide; and a second core coated to surround an outer surface of the first core and composed of the metal M³.
 14. The system according to claim 4, wherein the solid raw material module further comprises an oxidation-preventing layer surrounding the shell layer and serving to prevent oxidation of the metal contained in the core layer and/or the shell layer.
 15. (canceled)
 16. The system according to claim 1, wherein the metal M¹ is one selected from the group consisting of Ti, Zr, Hf, W, Fe, Ni, Zn, Co, Mn, Cr, Ta, Ga, Nb, Sn, Ag, La, Ce, Pr, Nd, Nb, Pm, Sm, Eu, Al, V, Mo, Gd, Tb, Dy, Ho, Er, Tm, Yb, Ac, Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md and No.
 17. The system according to claim 1, wherein the metal M² is at least one selected from the group consisting of Cu, Ni, Fe, Sn, Zn, Pb, Bi, Cd, and alloys thereof.
 18. The system according to claim 1, wherein the metal M³ is at least one selected from the group consisting of Ca, Mg, Al, and alloys thereof.
 19. The system according to claim 1, wherein the flux comprises a molten halide salt of at least one metal selected from the group consisting of alkali metals and alkaline earth metals.
 20. A method of reducing a metal oxide to metal M¹, the method comprising: providing a cell; introducing a liquid flux into the cell; introducing metal M¹ and metal M² forming a eutectic phase with each other, thereby producing a liquid metal crucible having a specific gravity higher than that of the flux and accommodated in the cell while forming a layer under the flux without being mixed with the flux; moving a solid raw material module comprising a metal oxide, metal M² and reducing metal M³ to the cell until it reaches the liquid metal crucible through the flux; and obtaining a liquid metal alloy comprising metal M¹ derived from the metal oxide of the solid raw material module and metal M².
 21. The method according to claim 20, further comprising obtaining metal M¹ by electrorefining the obtained metal alloy comprising the metals M¹ and M².
 22. The method according to claim 20, wherein oxide M³ _(a)O_(b) is produced as a by-product in moving the solid raw material module and/or obtaining the liquid metal alloy, and the oxide M³ _(a)O_(b) has a lower specific gravity than that of the flux, and the method further comprises continuously collecting the by-product M³ _(a)O_(b) forming a layer on the flux, and adding and mixing M¹ _(x)O_(z) with the collected M³ _(a)O_(b), thereby producing a metal oxide expressed as M¹ _(x)M³ _(y)O_(z) derived from the by-product M³ _(a)O_(b) and the added M¹ _(x)O_(z).
 23. A metal alloy obtained by the method according to claim
 20. 24. A metal obtained by the method according to claim
 21. 25. (canceled) 